Trifloxysulfuron controls green and false-green kyllinga more effectively than halosulfuron. Studies were conducted to evaluate the absorption, translocation, and metabolism of 14C-halosulfuron and 14C-trifloxysulfuron when foliar applied to green and false-green kyllinga. No differences were observed between the two kyllinga spp. with regard to absorption, translocation, or metabolism of either herbicide. The majority of 14C-halosulfuron and 14C-trifloxysulfuron was absorbed by 4 h, with an accumulation of 63 and 47% radioactivity, respectively. Accumulation of both herbicides occurred in the treated leaf and the primary shoot from whence the treated leaf was removed, with minor accumulation occurring in the roots and newly formed rhizomes. Of the total amount of 14C-halosulfuron absorbed into the plant, 77% remained in the form of the parent compound compared with 61% of 14C-trifloxysulfuron. The parent compound was distributed mainly in the treated leaf and primary shoot, whereas polar metabolites were concentrated in the roots and rhizomes. Nonpolar metabolites of 14C-trifloxysulfuron accumulated in the treated leaf and primary shoot. These data indicate that absorption, translocation, or metabolism could not explain the variation in green and false-green kyllinga control between halosulfuron and trifloxysulfuron.

‘Mars’, ‘Lemont’, and ‘Tebonnet’ rice were treated in 1987 and 1988 with fenoxaprop, sethoxydim, and haloxyfop at 3- to 4-leaf preflood and at panicle initiation postflood growth stages. Rice stunting and stand loss generally were greater from herbicides applied at postflood than at preflood with all cultivars. Mars yields were reduced from 11 to 23% by fenoxaprop at 168 and 336 g ha−1 applied preflood or postflood. Only postflood applications of fenoxaprop reduced Lemont and Tebonnet yields. Sethoxydim at 112 and 224 g ai ha−1 reduced yields of all cultivars when applied postflood, but yields were not reduced with preflood applications. Haloxyfop preflood at 140 g ai ha−1 and postflood at 70 and 140 g ai ha−1 reduced yields of Mars both years compared to the untreated control. Yields for Lemont and Tebonnet were reduced both years only when haloxyfop was applied postflood at 140 g ha−1.

Field experiments were conducted in 1985 and 1986 under eight environments to evaluate the population dynamics of a range of silverleaf nightshade densities and to measure the effects of those populations on cotton lint yield. Dry weed weights of silverleaf nightshade stands were influenced by growing conditions among years, but were positively related to initial densities as long as 2 yr after establishment. Stem numbers increased as initial densities and stand age increased. A negative linear relationship existed between cotton lint yield and weed biomass and between cotton lint yield and stem number from both 1- and 2-yr-old weed stands. For each 1 kg/10 m of row increase in dry weed weight from 1- and 2-yr-old stands, a 9 and 21% lint yield loss/ha was predicted, respectively. For each stem/10 m of row, a 0.35 and 0.31% yield loss was predicted, respectively. Late-planted cotton was less vulnerable to yield reductions by silverleaf nightshade; however, its yield potential was also less.

The tolerance of bermudagrasses ‘Tifway’, ‘Tifgreen’, ‘Tifdwarf’, ‘Ormond’, and ‘Floraturf’ [Cynodon dactylon (L.) Pers.] cultivars to repeated annual applications of oxadiazon [2-tert-butyl-4-(2,4-dichloro-5-isopropoxyphenyl)-Δ2-1,3,4-oxadiazolin-5-one] and butralin [4-(1,1-dimethylethyl)-N-(1-methylpropyl)-2,6-dinitrobenzenamine] were evaluated over a 4-yr period. Oxadiazon treatments did not affect growth characteristics of these bermudagrasses. Butralin caused moderate to severe reduction in sod density. ‘Tifway’, ‘Ormond’, and ‘Floraturf’ were more tolerant to butralin than ‘Tifgreen’ and ‘Tifdwarf’.

Replacement series and growth analysis experiments under greenhouse and field conditions quantified and explained the competitive ability of wild oats and barley. Barley was a stronger competitor than wild oats under greenhouse and field conditions. The reciprocal yield approach showed that barley's intraspecific competition was 7.3 times greater than its interspecific competition with wild oats when calculated on a dry weight per plant basis. When leaf area per plant was the yield variable, barley's intraspecific competition was only 2.4 times greater than its interspecific competition. The difference was explained by wild oats' higher leaf area ratio. Barley had a greater leaf area, root and shoot biomass, absolute growth rate, and shoot-root ratio than wild oats, but wild oats' leaf area ratio was always higher. No differences were detected in relative growth rate and net assimilation rate.

Growth and regeneration of field horsetail (Equisetum arvense L. ♯ EQUAR) were investigated under field and greenhouse conditions. Field horsetail produced shoots during the entire summer but its growth was detrimentally affected by the presence of corn (Zea mays L.). Field horsetail regrew as well whether it had been hoed once or 16 times during the growing season; and during the subsequent summer, there were no significant differences between horsetail growth in plots kept weed-free the previous growing season and in plots where field horsetail grew freely. In the greenhouse, rhizome segments 1 cm long placed 15 cm deep easily regenerated plants. Simulations were done using the growth and regenerative ability of field horsetail to estimate its potential spread in agricultural fields. The results of these simulations indicate that this weed can infest a hectare within 6 yr of its introduction into a new area.

Soil samples representing a time series from forest before clearing to land in cultivation for 20 yr were taken near Delta Junction, AK, to determine the changes that occur in the soil seed bank following clearing and agricultural use. The total number of seed was initially low in the forested sites but increased after 3 to 5 yr in cultivation due to seed production by native colonizers and by introduced weeds such as common lambsquarters (Chenopodium album L. ♯3 CHEAL). Viability of seed of several mature forest species was low. There was a shift from a seed bank dominated by native species in the forest and newly cleared sites to a seed bank dominated by introduced colonizers in the older fields. Although seed of introduced species increased in importance over time, native species, especially native colonizers, continued to be an important component of the seed bank throughout the 20-yr period represented.

Seed were harvested from soybean [Glycine max (L.) Merr. ‘Cutler’] plants treated at flowering and pod filling with (2,4-dichlorophenoxy)acetic acid (2,4-D), 4-(2,4-dichlorophenoxy)butyric acid (2,4-DB), and 3,6-dichloro-o-anisic acid (dicamba). Progeny of plants treated at flowering with 2,4-D and 2,4-DB or at pod filling with the lowest rate were normal. When higher rates were applied at pod filling, these phenoxy herbicides caused appreciable injury to the progeny in the form of reduced emergence and dry weight and malformed unifoliate leaves. Dicamba was much more injurious to the progeny of treated plants than 2,4-D and 2,4-DB. Even at low rates dicamba caused reduced germination, emergence, and dry weight and malformed first trifoliate leaves.

Understanding spatial distribution has become increasingly important in weed science. Seed dispersal, both between and within agricultural fields, is an important component of weed spatial distribution. Analysis of the effect of dispersal between fields has been relatively neglected in theoretical studies of weed population dynamics. In this paper, we present a simple landscape-level model of the influence of seed dispersal on winter wild oat population dynamics between fields. In the model, two fields are interconnected, with seeds being carried from one field to another as would occur when seeds are carried by field equipment or in irrigation water. The model is intended to characterize the effect of field-level weed management decisions on landscape-level weed population dynamics. Three scenarios were studied. The first employed no control measures in either field. The second employed annual application of herbicides in field 1 with field 2 receiving no treatment. In the third scenario, an herbicide application took place in field 1 only if the weed population in that field exceeded an action threshold. In the first scenario, the net result of the immigration and emigration processes determined the increase or decrease of the stable plant population. In the second scenario, weeds in the controlled field (field 1) were not driven to extinction as might be expected. The weed populations grew for practically all the dispersal parameter space. Each change in the parameter's values produced a new stable equilibrium. This situation might correspond to a multiplicity of stable states. The uncontrolled field (field 2) experienced an indirect control effect due to the use of control measures in field 1. In the third scenario, we observed an interesting behavior of the populations in both fields. The population in field 1 was not driven under the economic threshold, and both fields showed complex dynamics within defined combinations of migration and emigration values.

Among 12 of the more effective surfactants selected from a preliminary experiment, there were few differences in their ability to increase atrazine toxicity to large crabgrass [Digitaria sanguinalis (L.) Scop.] and sorghum (Sorghum vulgare Pers.), or in toxicity to large crabgrass and sorghum when used alone. Surfactants per se showed some toxicity to large crabgrass and sorghum. As large crabgrass increased in size, it became less susceptible to injury from atrazine plus dodecyl ether of polyethylene glycol (Surfactant WK). Early spraying with atrazine plus Surfactant WK reduced the top-growth yield of six grass species.

In field studies, atrazine alone and surfactants plus atrazine applied at the 1-in weed height gave very good early weed control. Field plots treated at the 4-in weed height generally produced more weed top-growth and less sorghum than plots treated at the 1-in height. Only the preemergence atrazine treatments gave weed yields that were equal to treatments that included handweeding. Sorghum injury from the surfactants was evidenced by reduced stover yields.

Atrazine plus surfactant toxicity to emerged large crabgrass, grown in growth chambers, increased with higher relative humidities. This was not true for sorghum. The 1% surfactant mixtures generally had a lower surface tension than the 0.1% concentrations although the differences were not great. Surface tension was not the main factor governing the phytotoxicity of foliar-applied atrazine solutions. Translocation of atrazine from sorghum and large crabgrass leaves dipped in a C14-ring-labeled atrazine solution was very slight or nonexistent.

The sensitivity of giant duckweed (Spirodela polyrhyza (L.) Schleid), common duckweed (Lemna minor L.), Wolffiella floridana (Smith) Thomp., azolla (Azolla caroliniana Willd.), and watermeal (Wolffia columbiana Karst.) to diquat (6, 7-dihydrodipyrido [1,2-a:2’,1′-c] pyrazidiinium salt) and paraquat (1,1′-dimethyl-4,4′-bipyridinium salt) was in that order (most to least sensitive). The rapidity of chlorosis in giant duckweed was directly related to the light intensity. The light quality having the most effect on the activity of diquat and paraquat on duckweed was red (600+ millimicrons), and green (510–560 millimicrons) had the least effect.

A growth cabinet study was conducted to evaluate the effect of temperature on the distribution of several herbicides in quackgrass. Uniformly labeled 14C-sucrose and the radiolabeled herbicides glyphosate, sethoxydim, cloproxydim, the butyl ester of fluazifop, the methyl ester of haloxyfop, and the ethyl ester of quizalofop were applied to quackgrass grown at three day / night temperature regimes (10/5, 20/15, and 30/25 C). Seven days after treatment the plants were harvested, lyophilized, and later sectioned, mapped, and oxidized in preparation for 14C quantification. Quackgrass rhizome growth was more vigorous at 20/15 than 30/25 or 10/5 C. of the herbicides tested, haloxyfop was the most inhibitory to rhizome growth. Temperature increases from 10/5 to 20/15 or from 20/15 to 30/25 C resulted in more translocation to shoots. Increasing temperature had various effects on translocation to rhizomes depending on the chemical applied. At all three temperature regimes, more 14C was recovered from distal than basal buds in plants treated with 14C-sethoxydim. In contrast, at all three temperature regimes, similar amounts of 14C were recovered from the distal and basal buds of plants treated with 14-C-sucrose.

Twelve analogs of the strigol D-ring (butenolides) were evaluated at 0.1 mM as germination regulators of 26 weed and crop seeds. In general, the butenolides either inhibited or did not affect monocot germination, although the hydroxybutenolide stimulated germination of dormant cheat and nondormant perennial ryegrass and sorghum seeds. The response rate for dicots was greater but still primarily inhibitory. Dormant lettuce seeds (both light-sensitive and light-insensitive), Palmer amaranth, and redroot pigweed showed several significant responses, both inhibitory and stimulatory. Some of the butenolides that had not been previously assayed against witchweed were tested as 10-4 to 10-11 M dilution series. The allyloxybutenolide demonstrated activity comparable to strigol (30 to 60% germination at 10-6 to 10-11 M); the sec-butyloxybutenolide was somewhat less active (20 to 40% germination) but still effective at 10-9 M. In general, the butenolide analogs of strigol were less effective in stimulating seed germination than the multiring analogs.

Field and greenhouse experiments were conducted to observe and measure the phytotoxicity of glyphosate [N-(phosphonomethyl)glycine] to potatoes (Solanum tuberosum L. ‘Russet Burbank’) applied at different stages of growth and to determine if glyphosate was translocated into the original mother tuber (seed piece) prior to initiation of daughter tubers. Glyphosate injury symptoms appeared 1 day after treatment and at least 50% of the foliage was necrotic within 7 days. Significant decreases in plant height, shoot and root dry matter content, and daughter tuber production were recorded at the 0.28- and 0.56-kg/ha rates. Higher rates of 1.12 and 2.24 kg/ha did not give further significant reductions. Maximum levels of 14C activity accumulated in all plant parts 4 days after application of the 14 C-glyphosate. The 14 C-glyphosate accumulated primarily in the apical meristem and roots. Extremely low levels of 14 C activity were detected in the mother tuber regardless of plant age when treated. Phytotoxic effects on the eyes of the mother tuber increased in severity with increasing rates of glyphosate as evidenced by abnormal sprouting.

Long-term absorption of 14C-atrazine [2-chloro-4 - (ethylamino) - 6 - (isopropylamino) - s - triazine] by excised velvetleaf (Abutilon theophrasti Medic.) roots was investigated. A rapid, initial phase of absorption was followed by a long-term, linear phase of absorption, which resulted in atrazine accumulation in the tissue to concentrations above the external solution. This accumulation did not result from metabolism of the atrazine. Aging of the excised tissue appeared to be responsible for the accumulation. Dinitrophenol, but not cyanide, inhibited accumulation. Atrazine accumulation decreased as the temperature during absorption was decreased. The Q10 the absorption rate was ca. 2.5 between 15 and 25 C. Efflux of atrazine from the tissue occurred in two phases. At 25 C, loss of 80% of the atrazine occurred in 20 min. At 4C, the rate of this efflux was slower and the phase lasted longer. At both 4 and 25 C a second, slower phase of efflux occurred through at least 6 h and resulted in loss of equal amounts of atrazine at the two temperatures. These experiments show that, in addition to rapid absorption of atrazine to a passive equilibrium, excised velvetleaf roots accumulate atrazine by a metabolically dependent process over long periods of time.

Growth and reproductive potential of individual yellow nutsedge plants were examined under two nitrogen levels and three soil moisture regimes. Irrigation levels were selected on the basis of irrigating at soil water potentials of −20, −50, and −80 kPa. Yellow nutsedge patch expansion was measured using digital images to determine ground cover, while plots were subsampled to estimate total shoot and tuber production. High nitrogen increased shoot production in 2004. When plots were irrigated at a soil water potential of −20 kPa, a single yellow nutsedge plant produced 3,000 and 1,700 shoots and 19,000 and 20,000 tubers in 2003 and 2004, respectively. Patch expansion at −20 kPa was exponential, with the greatest expansion occurring between the middle of July and mid to late August. This research demonstrates that the vegetative and reproductive potential of yellow nutsedge is greatly influenced by irrigation level. With such extensive growth and reproductive potential, management strategies for yellow nutsedge should focus on prevention, early detection and containment, early treatment, and integration of control strategies to reduce its competitiveness and spread.

Striga hermonthica reduces cereal yields in large areas of sub-Saharan Africa. Recent research has shown that transplanting maize seedlings that are more than 15 d old reduced Striga emergence and improved crop yields. Field experiments were conducted in 1998 and 1999 to determine whether maize varieties with different maturity periods and susceptibility to Striga parasitism respond similarly to transplanting. There was a considerable difference in Striga emergence between varieties in direct-seeded maize, but transplanting clearly reduced Striga emergence for all varieties. Transplanting of maize in plastic tubes gave the best Striga control until 8 wk after transplanting, whereas transplants from nurseries provided better season-long control. Transplanting improved grain yields 50 to 100% compared with direct seeding for three of the four varieties tested. Only the early-maturing variety ‘Morogoro’ had lower yields with transplanting than with direct seeding, indicating that transplanting caused more stress on the plants than was alleviated by the lower Striga infestation. The two varieties (‘Pioneer 3251’ and ‘H622’) most susceptible to Striga parasitism profited the most from transplanting, and the concomitant reduction in Striga induced stresses. Increases in productivity because of transplanting were associated with increases in biomass or harvest index. Transplanting of different maize varieties under rain-fed conditions has proven to be a biologically efficient method to improve maize yield and reduce Striga infestation within one season.

A stochastic simulation modeling framework was developed for measuring the impact of weed management technologies in terms of their risk and efficacy. The framework explicitly accounted for the variability of environmental conditions, which underpins risk, and its effect upon the weed population dynamics and crop yields. It was applied to wild oat and wild radish in spring wheat as case studies. Technologies considered included a postemergence herbicide, preseeding tillage, increased crop density, and a selective spray-topping (seed-head sterilizing) herbicide. All stages of the weed life cycle were specified as random variables on the basis of triangular probability distributions, which either were derived from daily environmental conditions or specified as a subjective probability distribution. By using probability density functions the study identified the risks to changes in the weed seed bank and weed density associated with various integrated weed management strategies. This approach not only quantified the probabilities associated with the different outcomes, but also identified how the probability distributions of outcomes were changed as a result of different technology combinations used in weed management strategies. For instance, strategies involving a selective spray-topping herbicide to control seed rain not only resulted in lower seed banks, but the range in possible values was considerably reduced, implying lower likelihood of a population increase. The use of such a risk framework not only benefits weed scientists in terms of designing more effective weed management technologies, but can also assist farmer adoption by being able to quantify the probabilities of success and failure of a technology.

Wild mustard (Sinapis arvensis L.) is a weed that frequently infests winter wheat (Triticum aestivum L.) fields in Golestan province, Iran. Tribenuron-methyl (TM) has been used recurrently to control this species, thus selecting for resistant S. arvensis populations. The objectives were: (1) to determine the resistance level to TM of 14 putatively resistant (PR) S. arvensis populations, collected from winter wheat fields in Golestan province, Iran, in comparison to one susceptible (S) population; and (2) to characterize the resistance mechanisms and the potential evolution of cross-resistance to other classes of acetolactate synthase (ALS)-inhibiting herbicides in three populations (AL-3, G-5, and Ag-Sr) confirmed as being resistant (R) to TM. The TM doses required to reduce the dry weight of the PR populations by 50% were between 2.2 and 16.8 times higher than those needed for S plants. The ALS enzyme activity assays revealed that the AL-3, G-5, and Ag-Sr populations evolved cross-resistance to the candidate ALS-inhibiting herbicides from the sulfonylureas (SU), triazolopyrimidines (TP), pyrimidinyl-thiobenzoates (PTB), sulfonyl-aminocarbonyl-triazolinone (SCT), and imidazolinones (IMI) classes. No differences in absorption, translocation, or metabolism of [14C]TM between R and S plants were observed, suggesting that these non-target mechanisms were not responsible for the resistance. The ALS gene of the R populations contained the Trp-574-Leu mutation, conferring cross-resistance to the SU, SCT, PTB, TP, and IMI classes. The Trp-574-Leu mutation in the ALS gene conferred cross-resistance to ALS-inhibiting herbicides in S. arvensis from winter wheat fields in Golestan province. This is the first TM resistance case confirmed in this species in Iran.

Experiments were conducted to determine the amount of time required for waterhemp to produce mature seeds after pollination. Female waterhemp plants were pollinated over a 24-h time period and then isolated from males. Two branches, each containing at least 500 flowers, were harvested from each female at the time of the initial pollination, designated as 0 d after pollination (DAP), as well as at multiple other times after pollination up to 62 DAP. One branch from each harvest was stored at 30 C for 48 h, while the other branch was stored at −20 C for 48 h. Branches were then stored at room temperature until all harvests were complete, at which time seeds from each branch at each time after pollination were collected, weighed, and stratified. Germination tests were then conducted to determine the time at which seeds become viable after pollination. Seeds that had not germinated by the end of the germination tests were subjected to tetrazolium testing for viability. Germination tests were also conducted on nonstratified seeds to investigate changes in seed dormancy that were expected to occur over the amount of time the seeds were allowed to remain on the plants. Seeds stored initially at 30 C postharvest became viable 7 to 9 DAP, whereas seeds stored initially at −20 C postharvest did not become mature until 11 DAP. Seed coat color was white soon after pollination and became dark brown to nearly black by 12 DAP, and seed weight increased until 12 DAP. Tetrazolium tests for seed viability correlated well with the germination tests. Germination tests on nonstratified seeds indicated that dormancy level was initially high in the population used, but began to decrease between 15 and 30 DAP. Results of this study have implications both for waterhemp management and research.